In the UMTS (Universal Mobile Telecommunications System) network, for the purposes of further increasing high-speed data rates, providing low delay and so on, long-term evolution (LTE) has been under study (non-patent literature 1). In LTE, as multi-access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used on the downlink, and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used on the uplink.
In the LTE system, uplink signals are mapped to adequate radio resources and transmitted from a user terminal to a radio base station apparatus. To be more specific, uplink user data is transmitted using an uplink shared channel (PUSCH: Physical Uplink Shared Channel). Also, the PUSCH is used when uplink control information (UCI) is transmitted with uplink user data, and an uplink control channel (PUCCH: Physical Uplink Control Channel) is used when uplink control information alone is transmitted.
In uplink control information (UCI), a delivery acknowledgment (ACK/NACK) in response to a downlink shared channel (PDSCH: Physical Downlink Shared Channel)), a scheduling request, channel state information (CSI) and so on are included (see, for example, non-patent literature 2). The channel state information (hereinafter referred to as “CSI”) refers to information that is based on the instantaneous downlink channel state, and includes, for example, channel quality information (CQI), the precoding matrix indicator (PMI), the rank indicator (RI) and so on. This CSI is reported from a user terminal to a radio base station apparatus periodically or aperiodically.
In response to a trigger from a radio base station apparatus, aperiodic channel state information (aperiodic CSI) is reported from a user terminal to that radio base station. This trigger (aperiodic CSI triggering) is included in an uplink scheduling grant (hereinafter referred to as “UL (uplink) grant”) (DCI format 0/4) that is transmitted by a downlink control channel (PDCCH: Physical Downlink Control Channel). The user terminal reports the aperiodic channel state information (hereinafter referred to as “A-CSI”) using the PUSCH designated by the UL grant, in accordance with the trigger included in the UL grant. Such reporting of A-CSI is also referred to as “aperiodic channel state information reporting” (aperiodic CSI (CQI/PMI/RI) reporting).